GMC Rut Runners

[gary miller]

Gene,
Thanks for the analysis of road rutting. I first experienced this driving
through Minnesota on a section of road traveled by heavy trucks --- must
have been a long section of poor road base. Anyway my wife almost got
seasick before I figured out what was going on and switched to the passing
lanes where rutting was not bad. Understanding the cause of the problem,
ie, different track of front vs rear wheels makes sense to me as to the
cause. To me it is not such a problem that I would chance any modifications
to my coach in an attempt to "fix" it.
Gary '77 Kingsley, Oregon.

> -- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] --
>
> I have been collecting information on this problem for some time, and I
> think the following describes what I have read.
>
> GMC Rut Runners
>
> The GMC jumps in and out of ruts in the road because the front wheels do
> not run in the same track as the rear wheels. This is an original
> design constraint of the GMC. Slack or looseness in the front
> suspension, steering and rear suspension aggravates this condition.
> Tires can also affect this condition.
>
> Extended wheels can correct the tracking. Machining a hub-centered
> adaptor for these wheels is needed because extended wheels available are
> not hub-centered but hang from the stud bolts. The load of the GMC
> wears these bolt holes and causes other problems. The extended wheels
> have been calculated to increase the loading on the hubs and bearings(
> already at maximum ) by 450% ! An interesting article in the GMC
> Motorhome News (September 1998) by Chuck Aulgur, Mechanical engineer,
> describes the technical aspects of this mounting. The results are not in
> at this time if this will significantly reduce the life of the front
> bearings, Hubs, etc. Users of these wheels say this eliminates the rut
> running.
>
> Four bagger modifications change the side to side roll stability of the
> GMC but do almost nothing for the tracking problem. The four baggers in
> essence remove the bogie action on the GMC and convert them into four
> independent wheels.
>
> My coach will jump ruts on a heavily rutted road. But it drives
> beautify and much better than the 30-foot SOB I rented several years ago
> .
>
>
> gene

 

[john dolan]

> My coach will jump ruts on a heavily rutted road. But it drives
> beautify and much better than the 30-foot SOB I rented several years
> ago

That possibly explains, to me, the apparent anecdotal inconsistancies
between replacing this or that suspension part, and solving or not
solving the rut jumping problem. One person replaces one part and
apparently solves his problem, while another person replaces all possible
contributing parts and does not solve his problem. It seems as though
worn parts make it worse but the degree of rutting, as well as other
factors make it more or less noticeable. Could some of those other
factors be: the response and speed of steering response of the driver,
cross winds, and possibly where you normally track on a rutted road?
- --

Regards,
John Dolan

jdolan

 

[hdavis]

>
>In a message dated 10/11/98 4:01:35 PM Central Daylight Time,

>
> have been a long section of poor road base. Anyway my wife almost got
> seasick before I figured out what was going on and switched to the passing
> lanes where rutting was not bad. Understanding the cause of the problem,
> ie, different track of front vs rear wheels makes sense to me as to the
> cause. To me it is not such a problem that I would chance any modifications
> to my coach in an attempt to "fix" it.
> Gary '77 Kingsley, Oregon. >>

The "rut problem" is there to some degree or another in every vehicle. The question is what extent and under what conditions
does the vehicle hop.

My wife's Fiero is pretty mild on most ruts most of the time. But so then is our GMC. My son's BMW is quite a bit more touchy
in ruts. Towing a trailer behind our International Taveler in Texas was a real experience. The truck by itself was easy to
move up and down the ruts. The trailer was a different story that moved the back of the truck.

Moving the tracking will only move the problem - there's another set of ruts in the country that will become a problem with
the new geometry. The only soultion is to not drive in ruts.

Arch -

It's amazing what folks will accept in coach performance just because the mileage has built up. And all the time they think
that something doesn't need to replaced or rebuilt in some number of miles. I've had big block chevy engines last over 300k
miles and not burn oil. But I've also had a newer chevy V6 last less than 60k mailes even with the right maintenance. My
natural assumption about a GMC is that everything needs rebuilding. It's a matter of when you can get to it.

Henry

Henry Davis Consulting, Inc / new product consulting
PO Box 1270 / product readiness reviews
Soquel, Ca 95073 / IP reviews
ph: (408) 462-5199 / full service marketing
fax: (408) 462-5198
http://www.henry-davis.com

 

[dick kennedy]

Any vehicle has serious problems on badly rutted lanes and we'll probably
never get agreement on how bad is bad. I've seen a few horror cases over the
years.

On the other hand a GMC with a handling problem is something to behold. On
mine the rubber bushings holding the sway bar had deteriorated. This
apparently allowed the sway bar to shift slightly as the vehicle swayed
which in turned changed the steering geometry slightly. It would go down the
road more or less straight but then any steering correction or road
irregularity that would shift the sway bar would create an oversteer
condition. The effect seemed to be equivalent to a fraction of a turn play
in the steering which would randomly lurch one way or the other. This
resulted in quite a bit of whip by the time it got amplified at the rear of
the coach. (At least this always kept me wide awake while I was driving)

I'd had it in two different shops for the problem before I finally figured
it out for myself then when I replaced those rubber bushings there was a
dramatic improvement in drivability. From watching the posts I think there
are a lot of people who have similar problems.

Mine is to the point where it drives OK but I wouldn't call it great. I've
owned two other class C's. They felt top heavy but they drove OK while the
GMC seems to have more of a tendency to follow the road. Both of these other
machines were low enough milage that they were basically new while the GMC
has been on the road for over twenty years. Handling is highly subjective.
What I've got is good enough to travel cross country without problems. I've
got six wheel alignment and all of that on my list of things to do but it's
not currently at the top.

As far as the difference in front to rear width being an issue I'm a
skeptic. Has anyone made the change without changing anything else at the
same time? I know mine always seems to be a little nicer and run a little
better just after I wash it. This type of thing is so subjective that I'm
not sure I could judge the difference unless there was a really dramatic
change. Then there's those flimsy bearings!

Keep talkin' I'm listening!

Dick

>>
>>In a message dated 10/11/98 4:01:35 PM Central Daylight Time,

>>
>>> have been a long section of poor road base. Anyway my wife almost got
>> seasick before I figured out what was going on and switched to the passing
>> lanes where rutting was not bad. Understanding the cause of the problem,
>> ie, different track of front vs rear wheels makes sense to me as to the
>> cause. To me it is not such a problem that I would chance any modifications
>> to my coach in an attempt to "fix" it.
>> Gary '77 Kingsley, Oregon. >>
>
>The "rut problem" is there to some degree or another in every vehicle. The
question is what extent and under what conditions
>does the vehicle hop.
>
>My wife's Fiero is pretty mild on most ruts most of the time. But so then
is our GMC. My son's BMW is quite a bit more touchy
>in ruts. Towing a trailer behind our International Taveler in Texas was a
real experience. The truck by itself was easy to
>move up and down the ruts. The trailer was a different story that moved the
back of the truck.
>
>Moving the tracking will only move the problem - there's another set of
ruts in the country that will become a problem with
>the new geometry. The only soultion is to not drive in ruts.
>
>Arch -
>
>It's amazing what folks will accept in coach performance just because the
mileage has built up. And all the time they think
>that something doesn't need to replaced or rebuilt in some number of miles.
I've had big block chevy engines last over 300k
>miles and not burn oil. But I've also had a newer chevy V6 last less than
60k mailes even with the right maintenance. My
>natural assumption about a GMC is that everything needs rebuilding. It's a
matter of when you can get to it.
>
>Henry
>
>Henry Davis Consulting, Inc / new product consulting
>PO Box 1270 / product readiness reviews
>Soquel, Ca 95073 / IP reviews
>ph: (408) 462-5199 / full service marketing
>fax: (408) 462-5198
>http://www.henry-davis.com
>
>